Something is wrong with the chapter 13.1 of 1x1/2.
The chapter order is "12" -> "13.1" -> "13.1 - +α" -> "13.2". If the problem was just that the "13.1" and the "13.1 - +α" were inverted in order in the main page or the fact that the reader joins them together while renaming the "13.1" to...